This article is continuation of the debate about the principles of the study of the Quran. It deals with its 5th principle according to that the understanding of the person who is engaged in studying the Quran must be coherent with the rules of reason & rationality. For the author, an accurate understanding of the Quran is possible only when its recitor does not abandon the reason and logical argumentationts as this is granted authenticity (hujjiyat) by Almighty Allah against humans and its validity is established by Quranic verses and traditions of the infallible imams. It is, however, not to say that Quran is subordinate to reason; since recitors’ understanding of Quran is a human reality; not Quran itself. In simple words, to follow the rules and principles of reason in understanding the Quran is not to juxtapose it with reason. Author also explains the nature of reason, its different types and their rules & Regulations and those prerequisites necessary to follow for understanding the Quran.